I dont know how to accept this

I have been dating a guy for 4 months and everything was going so well. We had a sleepover Wednesday night, talked all lovey together on Thursday, called me and told me I was on his mind before bed on Saturday and Sunday he told me his heart isn’t in it and he can’t do this. This is the first person I have genuinely liked and let in after almost 4 years of intentionally being single. I just can’t wrap my mind around someone changing their mind so quickly. I do think he hasn’t;t healed from his last relationship and once I became too real he needed to find an out. I obviously agreed to ending things because I will never beg for a man- but every single minute of the day I am flooded with wanting to call him and ask why. I am keeping myself busy and I am doing all the things for me- working out and moving daily, going to therapy, journaling. But holy shit, why does this almost feel harder than when my 3 year long relationship ended.